docs(ai-chat): guide for migrating an AI SDK route handler to chat.agent

docs/ai-chat/fast-starts.mdx

@@ -266,11 +266,14 @@ This is an **import-chain** problem, not a runtime one. A "we'll strip the execu
           ...helper.toStreamTextOptions({ tools: headStartTools }),
           model: anthropic("claude-sonnet-4-6"),
           system: "You are a helpful assistant.",
-          stopWhen: stepCountIs(15),
         }),
     });
     ```
 
+    <Warning>
+      Don't set `stopWhen` here. The spread pins it to `stepCountIs(1)`, and overriding it makes the handler run steps the agent is supposed to own — the handover then splices a stream that has already moved past step 1.
+    </Warning>
+
     <Tip>
       Use the **same model** on both sides (route handler and `chat.agent`) to avoid a tone or style shift between step 1 and step 2+. Your LLM provider keys stay server-side in your warm process — Trigger.dev never holds them in this design.
     </Tip>